The present invention generally relates to semiconductor devices and more particularly to a laser diode that oscillates at a visible wavelength.
In the optical information processing apparatuses such as bar-code readers or optical disk drivers, a source producing a coherent optical beam is used. Particularly, there are applications wherein the optical source is required to produce the optical beam with the visible wavelength. Conventionally, He-Ne gas lasers have been used for this purpose. On the other hand, efforts have been made to replace the bulky He-Ne lasers by compact laser diodes that consume less power.
In using the laser diodes for the optical beam source of these various optical information processing apparatuses, there exists a problem in that the optical beam produced from the laser diodes generally has the wavelength of red close to the edge of the visible band. Thereby, there is a demand for shifting the oscillation wavelength of the laser diode to the shorter wavelength side as much as possible. It should be noted that the visual sensitivity of human eyes decreases to one-tenth when the wavelength is increased from 623 nm to 660 nm. When the optical beam having a shorter wavelength is produced, an improved recognition of optical information such as images would be achieved with reduced output power of the laser diode. Further, the use of the shorter wavelength beam would result in the increased recording density in the optical recording media such as optical disks.
The oscillation wavelength of the laser diode can be shifted shorter when a material having a larger band gap is used for the active layer of laser oscillation. Thus, the use of materials having the band structure that facilitates the direct transition of carriers and simultaneously has a large band gap is being studied. For example, there is a proposal to use an active layer having a composition of (Al.sub.x Ga.sub.1-y).sub.0.51 In.sub.0.49 P for the active layer. However, the materials that contain aluminum tend to cause a problem of unwanted oxidation of aluminum during the fabrication process of the device. When the oxide of aluminum is formed in the active device, the oxide acts as a defect that induces the recombination of carriers and the efficiency of laser oscillation is deteriorated inevitably. It should be noted that Al is easily oxidized when there is oxygen during the fabrication process. This problem becomes even more serious when the temperature of deposition is reduced.
The oxidation of Al in the AlGaInP mixed crystal is reduced when a high deposition temperature is used. However, the high deposition temperature facilitates the diffusion of impurities that are doped in the semiconductor layers of the laser diode, and the use of such Al-containing active layer may invite the problem of unwanted doping of the active layer.
Even in the active layers that are free from aluminum, the foregoing composition of Ga.sub.0.51 In.sub.0.49 P causes a problem in that the Ga atoms and In atoms cause a spontaneous ordering to form an ordered structure wherein Ga atoms and In atoms are arranged alternately on the (100) plane. Thereby, a (111)-oriented GaP plane and a (111)-oriented InP plane appear alternately in the crystal structure of the GaInP mixed crystal.
Such a spontaneous ordering is believed to be a second-order phase transition and appears below the critical temperature of about 700.degree. C. It is suspected that the large difference in the atomic radius between In and Ga is the cause of such a phase transition. More particularly, the energy of the crystal lattice as a whole would be reduced when the large In atoms and the small Ga atoms are arranged alternately rather than distributed at random.
It should be noted that, when the foregoing ordering occurs, the band gap of GaInP decreases by about 90 meV. Thus, when the foregoing composition of Ga.sub.0.51 In.sub.0.49 P is used for the active layer of the laser diode, the oscillation wavelength becomes slightly smaller than the predicted oscillation wavelength. This effect is known as the "50 meV problem" (Gomyo et al. "Band gap energy anomaly in GaInP and spontaneous ordering" Ohyohbutsuri, vol.58, no.9, pp.1360-1367, 1989, in Japanese). Although this decrease of the band gap of 90 meV may seem insignificant, one should note that the oscillation wavelength of the laser diode is located close to the longer edge of the visible band of light, and the slight shift in the wavelength causes a significant effect as already noted.
FIG. 1 shows the relationship between the band gap and the composition of the active layer in the system of GaP-InP.
Referring to FIG. 1, the band gap that corresponds to the direct transition between both .GAMMA.-valleys of the valence band and the conduction band is represented by the solid line, while the band gap corresponding to the indirect transition between the X-valley of the conduction band and the .GAMMA.-valley of the valence band is represented by the one-dotted chain. Further, the band gap corresponding to the indirect transition between the L-valley of the conduction band and the .GAMMA.-valley of the valence band is represented by the broken line.
There, the solid line represents the principal transition for optical emission and it will be seen that the line is not exactly linear but convex in the downward direction at the intermediate composition. This is known as the bowing effect. The endmember GaP has a band gap (=2.81) much larger than the band gap (=1.35) of the endmember InP while the composition Ga.sub.0.51 In.sub.0.49 is the only composition that establishes a lattice matching with a GaAs substrate. In other words, only the GaInP mixed crystal that has this composition can be grown on the GaAs substrate as an epitaxial layer. At this composition, the band gap for the transition between the .GAMMA.-valleys has the value of 1.9 eV. As a result of the bowing effect, this value of the band gap is smaller than the linear interpolation by about 0.2 eV. It is believed that the foregoing spontaneous ordering contributes, at least to a certain extent, to this decrease of the band gap energy.
FIG. 2 shows the relationship between the band gap energy Eg and the compositional parameter x for the material (Al.sub.x Ga.sub.1-x).sub.0.51 In.sub.0.49 P. Similar to FIG. 1, the solid line designated as "E.GAMMA.c-E.GAMMA.v" represents the transition between the .GAMMA.-valleys of the conduction band and the valence band, the one-dotted chain designated as "EXc-E.GAMMA.v" represents the transition between the X-valley of the conduction band and the .GAMMA.-valley of the valence band, and the broken line designated as "ELc-E.GAMMA.v" represents the transition between the L-valley of the conduction band and the .GAMMA.-valley of the valence band.
As can be seen in FIG. 2, the value of the band gap can be increased by introducing Al into the GaInP active layer. However, Al causes the problem of unwanted oxidation as already noted.